Add attach_disk and dettach_disk in VM class.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add a method named 'sync' to VMXML class.

Lots of times to call vmxml.undefine() and vmxml.define(), to
make our config in vmxml take effect in vm.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add test for svirt attach disk.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>